This invention relates generally to computer networks, and more particularly to virtual router groups. A basic enterprise network architecture includes a router which functions as a gateway for hosts in a LAN or other access network which is terminated at the distribution layer. One drawback of this basic architecture is that the router presents a single point of potential failure. In particular, the hosts in the access network lose connectivity with devices outside the access network upon failure of the gateway router.
One solution to the single point of potential failure problem of the basic architecture is to implement multiple gateway routers. Each host can utilize various discovery protocols to determine which router to utilize as the first hop toward a particular destination. Consequently, in a network with two gateway routers having different IP addresses the hosts will respond to a failure of one gateway router by resolving the IP address of the other gateway router so that traffic can be routed beyond the access network after the failure. One drawback of this solution is that it imposes routing overhead on the hosts. Consequently, recovery from a gateway router failure may be undesirably slow. Moreover, an access network may include a large number of hosts so a considerable amount of host resources may be tied up by discovery protocols. Further, some hosts may not be able to support the discovery protocols or identify the alternate path.
Another solution to the single point of potential failure problem is implementing a virtual router using the Virtual Router Redundancy Protocol (VRRP). VRRP enables a group of gateway routers to function as a single virtual router with a set of one or more static virtual IP addresses. An election procedure is used to designate one of the routers of the VRRP group as the master router. In one implementation the master router is responsible for forwarding data packets sent to any of the IP addresses associated with the VRRP group. The master router also sends VRRP control packets (hello messages) to all of the hosts. Backup routers in the VRRP group process those VRRP control packets to determine if the master router has become unavailable for forwarding data packets. In particular, if a VRRP control packet is not received from the master router within a predetermined period of time since the previously received VRRP control packet then the master router is considered to be unavailable and an election of a new master router is prompted. The election is based on priority indicators associated with each router in the VRRP group, and a previously unavailable master router can regain master router status upon becoming available again. Network administrators tend to favor frequent VRRP control packet generation and correspondingly short counters so that an unavailable master router can be quickly detected to expedite fail-over. Because the hosts can utilize the same set of virtual IP addresses both before and after the failure of a gateway router, the single point of failure can be avoided via redundancy without forcing hosts to discover other gateway routers. Further, VRRP does not impose significant overhead on the hosts and can distribute traffic loads across multiple routers.